The present disclosure generally relates to a determining values corresponding to operational conditions for a hybrid or electric vehicle, and more particularly to determining the isolation properties in terms of capacitance and resistance corresponding to the operation of a battery system for an electric vehicle.
Hybrid or electric power systems provide an alternative to conventional means of vehicular motive power by either supplementing (in the case of hybrid electric vehicles (HEVs)) or completely replacing (in the case of purely electric vehicles) a traditional internal combustion engine (ICE). One form of such alternative vehicle is known as an extended range electric vehicle (EREV) that is part of a larger class of vehicles referred to as electric vehicles (EVs). In one embodiment of the EREV, primary electric drive is achieved with a battery (also known as a rechargeable energy storage system (RESS)) that acts as a direct current (DC) voltage source to a motor, generator or transmission that in turn can be used to provide the energy needed to rotate one or more of the vehicle's wheels. Once the electrical charge from the battery has been depleted, backup power may come from an ICE to provide auxiliary onboard electrical energy generation. The Chevrolet Volt is an EREV being manufactured by the Assignee of the present disclosure. Other vehicular configurations besides EREVs (including plug-in electric vehicles (PEVs)) may also benefit from the use of batteries to provide propulsive and other electric power. In the present context, an electrically-powered (or electrically-propelled) vehicle is one that derives a significant portion of its propulsive force from a battery, RESS or related electric source, even if the vehicle has EREV or HEV properties.
Various battery architectures may be employed to provide motive or related energy to an EREV, including nickel-metal hydride batteries, lead acid batteries, lithium polymer batteries and lithium-ion batteries. Of these, the lithium-ion battery appears to be particularly promising for vehicular applications. The high volumetric heat generation rate and generally passive construction of lithium-ion batteries provides both the durability and functionality needed to serve as a propulsion system for cars, trucks, buses, motorcycles and related automotive or vehicular platforms.
One consideration, irrespective of the battery form, is to control electromagnetic emissions from high voltage sources such as DC-DC converter, DC-AC inverter, electric motor and shielded or unshielded high power cables (as well as any other components connected to the high voltage bus) that are commonly used in EVs or EREVs. Capacitive devices are commonly used in those high voltage components to attenuate differential mode and common mode noises, thereby reducing emission levels and improving the quality of received signals. Some of these capacitors (named as X-capacitors) are for crossing battery positive and negative terminals, and are commonly used in those high voltage components, often specifically to limit differential mode noises. If the X-capacitors short-circuit, they run the risk of starting a fire.
Other capacitors are placed or naturally formed between either terminal of the battery system and vehicle chassis; these are named as Y-capacitors (or Y-cap for short), where Y-capacity takes into consideration a combination of both intentionally designed-in Y-cap devices and inherent (i.e., parasitic) Y-capacity formed by metal housings, cooling and related structure. As discussed herein, the total Y-cap can be used (by knowing the capacitance associated with built-in devices) to calculate the parasitic Y-capacity; changes in such value may be used (among other things) as an indication of battery health. Y-capacitance is an important property of a high voltage battery system in EV applications in that it may be used as indicia of the status of numerous system operational conditions, including those related to cooling and electronics measurement. As such, accurate detection of actual Y-cap values (as well as isolation resistance (RISO)) is beneficial; this additional RISO information is also valuable in the event a vehicle is involved in a collision. Specifically, post-requirements on RISO involve voltage-based quantities. For example, a minimum value of 500 ohms/volt may be required for electric or hybrid vehicle battery systems, or 100 ohms/volt for a fuel cell stack. Other additional specifications (such as those mandated by a particular vehicle manufacturer's internal requirements (for example, a minimum of 1 megaohm of electrical isolation)) may also be imposed on the system.
Both Y-cap or RISO values are susceptible to time-varying, nonlinear and uncertainty-based conditions. In addition, severe disturbances arising from varying environmental conditions may exacerbate already difficult data acquisition and measurement. These factors make it extreme challenge to develop an effective method of detecting the actual Y-cap and RISO values. Several concepts have been developed as a way to perform these tasks under non-ideal (i.e., so-called “real world”) conditions. One example uses a direct current (DC) shifting method to estimate the isolation resistance, while others employ active methods to obtain the isolation information through injecting low frequency sinusoidal wave signals. Nevertheless, to the inventors knowledge, none of these approaches are able to obtain both Y-cap and isolation resistance effectively, accurately and robustly.